Japanese Patent Laid-Open Application No. 8-20226 discloses an automotive air conditioning apparatus. This air conditioning apparatus includes a heat pump system which heats the air by utilizing the heat generated when a compressible refrigerant is compressed, and a forced water supply system which heats the air by utilizing heated water.
In such an air conditioning apparatus, the heat pump system is only operated when the outside air temperature is higher than a specified temperature. If the outside air temperature is lower than the specified temperature, the heat pump system is not operated alone, but the forced water supply system is complementarily operated in conjunction with the heat pump system to prompt the rate of heating.
However, in a very low-temperature environment, i.e., about -5.degree. C. or lower, the refrigerant of the heat pump system is unable to receive sufficient heat from the outside air. Therefore, the air conditioning apparatus could not exhibit good heating performance corresponding to the electric power consumption. This results in poor heating efficiency. In the case of electric motor cars in particular, if any part of the electric power accumulated in the battery is consumed for air conditioning purposes, the distance which the car can run is correspondingly reduced. Therefore, the development of an air conditioning apparatus which can exhibit particularly good power efficiency has been desired.
Futhermore, in the heat pump system, another problem is that the refrigerant in an uncompressed state will become liquefied under very low temperature conditions. When the liquefied refrigerant is fed to the compression chamber of the compressor, the compressor may be damaged with the result that the heat pump system will become inoperative.